1. Field of the Invention
This invention relates to spatial light modulation and electro-optical devices generally, and more specifically to high speed, liquid crystal diffractive beamsteering devices and adaptive optics.
2. Description of the Related Art
A high speed, non-mechanical beamsteering device finds applications in optical fiber and laser communications, laser radar or other fields which require fast adaptive optics. High switching speed, wide steering range, fine angular resolution and high optical efficiency are particularly desirable in such a device.
One conventional approach for high speed diffractive beamsteering exploits the electro-optical properties of liquid crystals (LCs). This approach is analogous to the use of phased-arrays to direct microwave radiation, and has been clearly explained in several publications: see, for example, Paul F. McManamon, Edward A Watson, Terry A. Dorschner and Lawrence J. Barnes, xe2x80x9cApplications Look at the Use of Liquid Crystal Writable Gratings for Steering Passive Radiation,xe2x80x9d Optical Engineering Vol. 32, No. 11, pp. 2657-2664, (November 1993); D. P. Resler, D. S. Hobbs, R. C. Sharp, L. J. Friedman and T. A. Dorschner, xe2x80x9cHigh Efficiency Liquid Crystal Optical Phased-array Beam Steering,xe2x80x9d Optics Letters, Vol. 21, No. 9, pp. 689-691 (May 1, 1996); Paul F. McManamon, Terry A. Dorschner, David L. Corkum, Larry J. Friedman, Douglas S. Hobbs, Michael Holz, Sergey Liberman, Huy Q. Nguyen, Daniel P. Resler, Richard C. Sharp, and Edward A. Watson, xe2x80x9cOptical Phased Array Technology,xe2x80x9d Proceedings of the I.E.E.E., Vol. 84, No. 2, pp. 268-298 (February 1996); and O. D. Lavrentovich, D. Subacius, S. V. Shiyanovskii, and P. J. Bos, xe2x80x9cElectrically Controlled Cholesteric gratings,xe2x80x9d SPIE Vol. 3292, pp. 37-43 (1998).
The principle behind diffractive beamsteering by liquid crystal phase shifting is illustrated in FIG. 1. For simplicity, unidirectional (single angle) steering is shown. An incident coherent optical beam 20 is shown by its equi-phase surfaces. If we consider a hypothetical prism 22 inserted into the beam path, we can see that such a prism would introduce a linear gradient of optical path delay (OPD) across the beam, shown by phase delay profile 24. Because the prism has thickness which varies linearly with displacement in the direction x, it introduces corresponding linear phase delay profile 24, with constant gradient. The introduction of constant gradient of phase delay results in refraction of the beam 20, so the resulting output beam has equiphase fronts 26, propagating in a new direction as shown by direction vector 28.
In the arrangement of FIG. 1 a phase shift of 2xcfx80 can be subtracted periodically from the phase front without influencing the far-field pattern produced (because it corresponds to exactly one wavelength of the light beam). Thus, to produce refraction equivalent to that produced by the OPD gradient 24, it is sufficient to introduce a periodic, sawtooth-like or xe2x80x9cfoldedxe2x80x9d phase profile as shown by the periodic OPD profile 30. The phase profile 30 is equivalent to that of 24 except that the phase is reset whenever the cumulative phase shift reaches 2xcfx80 or an integer multiple thereof. The sawtooth phase profile 30 is also essentially equivalent to that produced by a conventional blazed grating.
FIG. 2 shows a simplified reflection mode device which uses liquid crystals to produce phase shifts approximating the blazed grating profile shown in FIG. 1, to steer a coherent beam. The illustration is a simplistic idealization of that device described in U.S. Pat. Nos. 5,098,740 and 5,093,747 (to Dorschner et al. and Dorschner, respectively). A layer of nematic liquid crystals 40 is sandwiched between a reflective groundplane electrode 42 and an array of discrete, electrically distinct transparent electrodes 44. The elements of the electrode array 44 are electrically connected to a plurality of drive voltages which vary stepwise across the array according to a staircase-like voltage ramp. The variation in electrode potential produces a corresponding variation in electrical field intensity at points within the liquid crystal layer 40. Manifestly, the electric field will vary with position within the layer, in accordance with electrostatic principles, but the average field will vary across the device in an approximate staircase profile. The material of the liquid crystal layer 40 is a nematic liquid crystal with the property that its orientation is dependent upon the field strength locally applied; therefore, the effective refractive index of the LC (for a particular polarization) will vary with distance x; and the resulting phase delay introduced during light""s transit across the LC layer will also vary approximately as the staircase-like ramp 48.
To implement electrically controlled beam steering in the above described device, the reflective electrode elements of 44 are controlled through addressing electronics to allow application of pre-determined voltages to the elements as required to produce a sawtoothed optical phase delay function. If the voltages are controlled so that the phase delay is reset periodically by subtracting 2xcfx80, then the resulting sawtooth OPD profile 48 approximates a linear phase delay gradient across the device in the x direction. Comparing this function to the phase functions in FIG. 1, we can see that the resulting function approximates the phase delay gradient of a refractive prism. The effect of such a gradient, together with the reflective electrodes 44, is such that incident (polarized) radiation 20 is reflected at an adjustable angle xcex8, in relation to the voltage profile applied to the electrode array 44.
While the device of FIG. 2 seems to hold promise as a beamsteering device, it is limited in several important performance parameters. Most significantly, switching speeds currently achievable by this device at the important communication wavelength of 1.55 nanometer are limited to below approximately 500 Hz. This limitation results from the relatively slow relaxation of nematic crystals as they settle from driven to relaxed states. Attempts have been made to improve switching speeds by increasing the liquid crystal birefringence, thereby reducing the cell gap (the thickness of LC layer 40); however, any such increase in birefringence is accompanied by an increase in viscosity, which in turn increases the relaxation time of the LC.
In addition to slow switching speeds, conventional nematic LCs have weak elastic anchoring forces which forbid very high phase gradients. Such gradients would be particularly desirable for low grating pitch and high steering angles (pitches of less than approximately 5 microns).
Another problem with prior LC optical phased array steering devices is their undesirable departure from ideal sawtoothed OPD characteristics. FIG. 3 compares the ideal and actual OPD characteristics of a typical LC optical phased array beamsteering device. The OPD function 58 represents the idealized, desirable sawtooth pattern. Note that the reset portion 60 of the waveform is ideally vertical, which signifies that the phase is reset from 2xcfx80 to zero over infinitesimal distance in the x direction. This idealized characteristic is not realizable by physical LC devices. Waveform 62 represents a more realistic, attainable waveform. In practice, the gradient of the phase delay is limited by the finite xe2x80x9cfly-backxe2x80x9d distance 64. The optical efficiency of the real device is in inverse relation to the length of the fly-back distance 64. As this distance becomes longer, ever greater fractions of the input beam are diffracted into undesired grating modes (secondary modes or higher). The attainable fly-back distance is limited by the elastic anchoring forces of the liquid crystal and by the field gradients obtainable within the device. Consequentially, the optical efficiency of a real device is limited by the elastic anchoring forces of the liquid crystal and the electric fringe-field effects in the flay-back region.
Prior liquid crystal phased array beamsteering devices limit the available beamsteering angles to discrete angle increments. This results from the conventional pixel interconnection and drive schemes. Commonly, in a conventional LC array not all of the electrode elements are electrically independent; rather, every nth electrode is typically connected together to form a periodically repeating series of electrodes. For example, referring back to FIG. 2, electrodes 65a and 65b are electrically connected, as are the other electrodes which correspond in a periodic sequence. Not all of the interconnects have been illustrated, to preserve clarity in the drawing. This periodic interconnection scheme limits the available beamsteering angles. The addressable beamsteering angles are restricted to those that correspond to integer multiples of 2xcfx80 phase ramps across each electrode subarray. Although large subarrays can accommodate many possible integer factors (and thus many steerable angles), the steerable angle is still limited to discrete increments; it is not continuously variable. This limitation is discussed in Resler et al., cited above.
In view of the above problems, the present invention is a device and method for imposing a spatial phase modulation on a coherent light beam. The device and method are particularly useful for beamsteering, although they can also be used to impose other spatial light modulations on a beam.
In the device, an electrical exciting circuit produces a plurality of oscillating electrical excitations, at least two of which have independently controllable frequencies. A set of drive electrodes are distributed in an array, and connected so that each receives a respective one of the oscillating electrical excitations from the electrical exciting circuit. A liquid crystal material is arranged to receive the coherent light beam and is disposed in proximity to the set of drive electrodes so as to receive electrical excitations in local regions from the drive electrodes. The liquid crystal material has a dielectric coefficient (for at least one polarization) which varies in its local regions in relation to the frequency of the local electrical excitation received by those regions. The frequencies and preferably also the voltages of the excitations are controlled so as to produce a desired profile of the refractive index (for at least one polarization) and a corresponding optical phase delay profile for the coherent beam which traverses the liquid crystal layer.
In one embodiment, at least the top electrode is transparent and the invention includes a high-efficiency reflective groundplane, displaced behind the drive electrodes at a distance substantially equal to an integer multiple of one half wavelength for the wavelength of the coherent beam. The groundplane increases optical efficiency by increasing reflective area without destroying the phase coherency.
The invention can suitably be embodied as either a unidirectional or a bidirectional beamsteering device. In a unidirectional device the electrodes are preferably a linear array of elongated narrow stripes. Such a device is suitable for beamsteering in a plane. The bidirectional device includes a two-dimensional, (preferably rectilinear) array of addressable electrodes, capable of producing phase gradients in two independent directions simultaneously, thereby steering a beam in two independent angles, most preferably in orthogonal directions.